The present invention relates generally to a device used for chromatically dispersing lightwave signals in fiber optics, and more specifically to a wide-bandwidth chirped fiber Bragg grating that has a low delay ripple amplitude.
Modem communication system providers are striving to increase the capacity of their systems to satisfy the rapidly growing exchange of information around the world. Increasing the data rate of a single wavelength channel is one strategy to increase the throughput on optical fibers. However, this approach is limited in that the data rate for a single optical channel will eventually reach practical limitations. An important strategy to further increase the available bandwidth is to add multiple wavelength channels. Multiple wavelength systems are referred to as being wavelength division multiplexed (WDM).
Optical communications systems are available with single-channel data rates at 10 Gbit/s and faster. To accommodate the spectral bandwidth of these signals, the channels in a WDM system are commonly spaced at 100 GHz, or xcx9c0.8 nm in the 1550 nm wavelength range. A device would have to be useful over bandwidths greater than xcx9c0.8 nm to be a truly multi-channel device in these WDM systems. Ideally a device would operate over a full communications band of wavelengths, so systems could be designed for any WDM or modulation scheme without needing to accommodate a specific dispersion correction module. Current communications bands are defined by optical amplifier operating ranges; for instance, the xe2x80x9cCxe2x80x9d band covers xcx9c1530 nm to xcx9c1560 nm and the xe2x80x9cLxe2x80x9d band covers xcx9c1570 nm to xcx9c1610 nm.
In these optical communications systems, short pulses of optical energy are sent through optical fibers to transmit information. These optical data pulse are comprised of a spectrum of wavelengths. Generally speaking, an unchirped pulse of duration t has a spectral width of xcx9c1/t, e.g., a xcx9c1 nanosecond (10xe2x88x929 second) pulse has a xcx9c1 GHz (109 Hz) spectral width. As a pulse travels along standard singlemode fiber in the xcx9c1550 nm range, the shorter wavelength components travel faster than the longer-wavelength components. This effect, called chromatic dispersion, broadens the pulse to the point that it eventually interferes with neighboring pulses in a pulse train and introduces errors in the detected data stream. A number of solutions have been proposed for this problem, but only dispersion-compensating fiber (DCF) and chirped fiber gratings have been considered seriously as potential candidates for deployment.
Dispersion-compensating fiber has high levels of dispersion of opposite sign to that of standard fiber. To compensate for the dispersion introduced by an 80-km span of standard fiber, one would have to concatenate a xcx9c16-km length of DCF into the system. These compensation modules are bulky, and due to the fiber design, suffer high optical attenuation and increased optical nonlinear effects. However, DCF is used today since no serious alternative exists.
Fiber Bragg Gratings (FBGs) have emerged as a promising solution for dispersion compensation. An FBG is an optical fiber or other optical waveguide with periodic, aperiodic or pseudo-periodic variations of the refractive index along its length in the light guiding region of the waveguide. Gratings are usually written in optical fiber via the phenomenon of photosensitivity. Photosensitivity is defined as the effect whereby the refractive index of the glass is changed by actinic radiation-induced alterations of the glass structure. The term xe2x80x9cactinic radiationxe2x80x9d includes visible light, UV, IR radiation and other forms of radiation that induce refractive index changes in the glass. Typically an interferogram of UV radiation is made and then a photosensitive fiber is placed into it. The period of the resulting FBG in the fiber is the period of the interferogram scaled by the waveguides refractive index.
To function as a dispersion compensator, the grating period of an FBG is chirped to reflect lagging wavelengths before faster wavelengths, which must travel further into the grating before they are reflected. An optical circulator is used to separate the input of the device from the output. A dispersion compensating grating (DCG) module recompresses a data pulse that had been corrupted by chromatic dispersion, and optical system performance is enhanced. The longer the grating, the greater the DCG compression factor and the wider the bandwidth of the device.
As a practical matter, long length gratings for dispersion compensation are not available, since extreme tolerances must be maintained to manufacture quality long length gratings. Fabrication errors in chirped gratings create ripples in the group delay curve and thus inaccuracies in the dispersion correction. The impact of these ripples on optical system performance is poorly understood, but some system designers have predicted that these ripples must be less than xcx9c40 ps peak-to-peak for a DCG to be useful as dispersion compensators in most systems. However, the magnitude of the ripple needed to make a useful FBG dispersion correction device has not been verified. A ripple amplitude of xcx9c40 ps peak-to-peak can be caused by a 20% variation in the FBG UV-induced index change, a xcx9c0.3% dimensional change in a fiber core, or a xcx9c4 pm error in grating pitch. Given that the silicon-oxygen inter-atomic spacing in glass is xcx9c160 pm, it has been widely believed that holding these tolerances during grating inscription is not possible, and that fiber fabrication tolerances are limiting the quality of the gratings that they produce.
In 1995, a Swedish research group reported the fabrication of a long-length FBGs by stitching smaller FBGs together. A small grating was written, the fiber was translated by a grating period through a UV-interferogram with a high-precision linear stage, and then the fiber was irradiated again. This process was continued until a grating of the desired length was made. With their system, this group reported it fabricated gratings of up to 50 cm in length. Since this announcement, other groups have extended this work and have reportedly fabricated gratings up to 2.5-m-long. The range of motion of available high-precision staging has limited the length these FBGs.
Several groups have adapted stitching methods to make chirped long length gratings, but stitching errors have caused these gratings to have delay ripple amplitudes that are far too large for use as dispersion compensators in optical communications systems. To implement a stitching technique, one must have precise knowledge of a fiber location relative to the writing interferogram. The accuracy of location measurements is limited by the motion stage encoderxe2x80x94usually interferometer based, which is susceptible to several degradations, such as interpolator inaccuracies, noise in edge detection electronic circuitry, and random fluctuations in received interpolator-laser light.
Several feasibility studies have been completed where long-length FBGs, fabricated by stitching, have been used successfully at specific wavelengths as dispersion compensators in optical communication systems. Since the FBG delay ripple imposed very large distortion-derived system penalties at most wavelengths, the wavelength of the transmitting laser in the communication system had to be adjusted in these studies to obtain reasonable system performance.
A common procedure for determining chromatic dispersion of a device is the modulation-phase shift method, as described in Chapter 12 of Fiber Optic Test and Measurement (ed. D. Derickson, Prentice Hall PTR, N.J., 1998, ISBN #0-13-534330-5). The output of a narrowband, tunable optical source is intensity modulated and applied to the device under test. The transmitted (or reflected) signal is detected and the phase of its modulation is measured relative to the electrical modulation source. The phase measurement is repeated at intervals across the wavelength range of interest. The curve of the relative group delay is constructed by accumulating these group delay changes across the measurement wavelength range.
The group delay ripple is determined by fitting with least squares minimization a line or a low-order polynomial to the relative group delay curve, and then this polynomial is subtracted from the curve. The remainder of the subtraction is the delay ripple. Typically this ripple is considered as being xe2x80x9chigh frequencyxe2x80x9d ripple, i.e. ripple with a periodicity of less than the channel bandwidth of a communications system, and xe2x80x9clow frequencyxe2x80x9d ripple, i.e. ripple with a periodicity greater than that of a channel bandwidth. Herein high-frequency ripple will be considered as ripple with a periodicity of less than 80 pm (10 GHz @ xcx9c1550 nm), and low-frequency ripple will be ripple with a periodicity of greater than 80 pm. The high-frequency ripple adds an intra-pulse distortion to a communication signal that is difficult to correct, thus the high-frequency ripple is considered more critical than the low-frequency ripple, which merely adds a slight error to the dispersion correction.
Establishing a correlation between delay ripple amplitude and optical system performance has been confusing because different groups measure DCGs in different manners and, often, are not explicit about their measurement procedures.
Although several studies have demonstrated that DCGs could be used as dispersion compensators for a single communication channel, useful wide bandwidth devices have not been demonstrated, mainly because the delay ripple amplitude of these devices was too large. Widely chirped DCGs with several WDM channels operating across their bandwidth have been demonstrated, but because the wavelength of the transmitting laser in the communication system needs adjustment in all of these studies to obtain reasonable performance, these DCGs proved to have only very narrow sections of usable bandwidth across their entire bandwidth. If a wideband DCG with a large delay ripple is useful only over a very narrow range as a dispersion compensator, then the utility of this widely chirped device is lost. Others have demonstrated DCGs in laboratory systems where DCGs have been used to correct the dispersion in several channels of a communication system, but a DCG that covers only part of a communication band has limited appeal, since the system must be specially engineered to accommodate such a device. Gratings with a bandwidth of at least a third of a communications band (xcx9c10 nm) or half a band (xcx9c15 nm) have some appeal, since fewer accommodations for the device must be made to use it in a communications system than narrower devices.
Although DCF is used widely today to solve chromatic dispersion problems in high-speed optical communications systems, new DCF designs must attempt to match their dispersion and dispersion slope to be opposite to that of a given transmission fiber. But these DCF designs don""t exactly match the dispersion characteristics of their intended fiber and thus leave a residual dispersion that accumulates over multiple spans of transmission fiber. Since DCF designs can be complicated and difficult to manufacture, several transmission fibers do not yet have a matching DCF solution, and some fibers, due to their complex dispersion characteristics, seem unlikely that a perfectly matching DCF design is possible across a wide bandwidth.
There thus exists a need for wide bandwidth (i.e., greater than several WDM channel spacings and preferably a full communications band) fiber Bragg gratings that can compensate for the chromatic dispersion and dispersion-slope in lightwave communications systems across its bandwidth. There also exists a need in the art for a wide-bandwidth chirped fiber Bragg grating that has a low delay ripple amplitude (i.e.,  less than xc2x150 ps). These and other needs are met by the present invention, as hereinafter described.
In one aspect, the present invention relates to a device used for chromatically dispersing lightwave signals in fiber optics, and to a method for using this device to achieve chromatic dispersion. The device comprises a wide-bandwidth ( greater than 0.8 nm) chirped fiber Bragg grating that has a low delay ripple amplitude ( less than xc2x150 ps). These devices are useful as chromatic dispersion correction devices in high frequency (e.g., 10 Gbit/s) optical communications system. In contrast to previously known devices used for this purpose, these devices do not exhibit large fluctuations in system performance penalty across their bandwidth.
In another aspect, the present invention relates to a method and apparatus for fabricating long length FBGs. In accordance with this aspect of the invention, the fiber being written on is translated with xcx9c1 ppm (10xe2x88x926) velocity control. This is preferably accomplished by placing the fiber into a helical groove on a spool that is mounted to a rotary spindle. The spindle is rotated at a constant velocity, which is preferably regulated by a large flywheel. The groove on the spool acts like a thread on a screw so that, as the spindle turns, a linear stage is used to track the laser beam on the fiber, in a manner analogous to cutting threads on a lathe. Gratings that are tens of meters long can be fabricated with this method.